Question

I have on top entity with many child entities.

Now, I want implement FindAll() method where I’m using this pattern with querying all my top entities to List. That’s fine but I then get all my child objects with it, Alert: Select N+1 in nhib. profiler.

After I get all my objects I send them to FromMyDomainModel method to extract values to ViewModel I need.

   using (ITransaction transaction = session.BeginTransaction())
   {
       List<Property> data =
                   session.Query<Property>()//I don't need fetch
                   .ToList();
        transaction.Commit();
        return EntityViewModel.FromDomainModel(data);
    }

Maybe there is better pattern so please feel free to post.

update: mapping code
from mapping you can figure it my entity code

   public PropertyMap()
        {
            Table("Property");
            Id(x => x.Id).GeneratedBy.Identity();
            Map(x => x.Version);
            Map(x => x.Created);
            Map(x => x.Updated);
            Map(x => x.Views);
           ....
            Map(x => x.Price);            
            HasMany(x => x.Photos).KeyColumn("PropertyId").Cascade.All();
        }

 public PhotoMap()
        {
            Table("Photo");
            Id(x => x.Id).GeneratedBy.Identity();
            Map(x => x.ImageData).CustomSqlType("VARBINARY(MAX)").Length(160000).Not.Nullable();
            Map(x => x.ImageMimeType).Not.Nullable();
            References(x => x.Property).Column("PropertyId");

        }

Answer 1

To reduce the select n+1 problem you need to set the batch-size

NH3.2+ sets loading batch-size to 20 by default so if you can upgrade then I would recommend it. See here [NH-2593]

If you cannot upgrade and if are using XML mappings then do something like:-

<bag/set/list ... batch-size='50' ..>
...
</bag/set/list>

If you are not using XML mappings then please post a mapping example.

edit based on your updated mapping

Try this and reprofile, you should see your n+1 reduce by a magnitude of 50

HasMany(x => x.Photos).KeyColumn("PropertyId")
  .SetAttribute("batch-size", "50").Cascade.All();

Also you can set this batch-size in the configuration before you build your session factory so it applies to all your collection fetching.